The invention relates to abrasion resistant coatings and methods for their application, and more particularly to a well-adhered abrasion resistant coating for plastics.
Traditionally, glass lenses have been used in cameras, projectors, telescopes and other optical instruments. Recent developments have shown that lenses can be prepared from thermal plastics by injection molding. The primary advantages of producing lenses by injection molding are that the lenses have a low material cost, are light weight and only require the use of unskilled labor to be produced since the lenses are in a finished state when released from the mold. Such injection molded lenses are highly resistant to shattering and do not require any subsequent milling, grinding or polishing after the molding step. Such lenses are, however, not durable, and tend to scratch easily under routine cleaning.
One way of protecting the plastic lenses is to coat them with an abrasion resistant material. The problem has been to find a suitable material which is both abrasion resistant and yet durable and well-adhered to the plastic. Many suitable coatings such as the type of coatings which are put on glass to act as anti-reflection coatings are difficult to put onto an injection molded plastic lens because of the requirements of the coating process. For example, glass lenses are typically coated with a single layer anti-reflection coating of magnesium fluoride (MgF.sub.2). Magnesium fluoride is deposited on the glass lens by vapor deposition, that is, by vaporizing magnesium fluoride in a vacuum chamber and then allowing the vapor to contact the heated lens. The lens must be heated to approximately 300.degree. C. This elevated substrate temperature is required to improve adhesion and durability of the magnesium fluoride coating to the glass surface.
Depositing magnesium fluoride on plastic lenses by vapor deposition is unsatisfactory because thermal plastics generally cannot withstand the high temperatures required for satisfactory adhesion and durability of the magnesium fluoride coating. Also, such coatings tend to show restricted performance during environmental testing due to poor adhesion.
If a soft magnesium fluoride coating is deposited on glass, then the standard procedure requires baking of the coated glass element at a temperature between 300.degree. C. and 500.degree. C. Plastics suitable for optical use are not able to maintain dimensional stability and often oxidize at these temperatures and would be destroyed by the baking process.
Other coating processes on plastic for increased durability of the surface have been attempted by dipping the substrate into a solution of the coating material and then removing the substrate. See, for example, U.S. Pat. No. 3,953,115. The problem with coating in this manner is that there is virtually no control over the film thickness. The films do not shown improved durability at thicknesses less than 1 micron and are too thick and non-uniform to be of use as an optical coating on the involved geometry of a lens. It is known to apply similar coatings to glass by plasma polymerization employed as a light guide, see for example, U.S. Pat. No. 3,822,928, but the problem of obtaining a well-adhered coating on an optical plastic substrate has remained until now.